Richard Burts Mon, 04/16/2007 - 08:25


As far as I know there is not an absolute limit on the number of EBGP sessions. It is mostly a question of the resources needed for each neighbor (how many routes, etc) and the resources available on the router.



s-durando Mon, 04/16/2007 - 08:44


so if I have for example a limited amount of routes (less than 5) for each BGP neighbor, how can I compute router's resources needed for each session?

Many thanks


mohammedmahmoud Mon, 04/16/2007 - 09:05

Hi Stefano,

You can use "show ip bgp neighbor " to check the amount of memory consumed by the routes received from a certain neighbor.


Mohammed Mahmoud.

s-durando Tue, 04/17/2007 - 00:45

Thank you Mohammed.

Anyway my issue is extremely practical: if I have for instance a 2851 router at headquarter site and this router has to support N x GRE tunnels to remote sites along with N x BGP neighborship, how can I determine N with a certain accuracy?



Danilo Dy Tue, 04/17/2007 - 00:53


For your 2851 in HQ, you will create 1 GRE Tunnel for each Remote GRE Capable Router. Also, you will establish 1 BGP Neighborship for each Remote BGP Capable Router.

Therefore, N in GRE Tunnel is the number of Remote GRE Capable Router, while N in BGP Neighborship is the number of Remote BGP Capable Router.

I say "Capable" because not all router model and IOS support IP GRE and BGP.

s-durando Tue, 04/17/2007 - 01:00

I would like to determine the maximum 'N' supported, with N the number of remote sites connected with a GRE tunnel to HQ site and with 1 BGP neighborship with HQ router



mohammedmahmoud Tue, 04/17/2007 - 03:07

Hi Stefano,

The number of interfaces supported by a router is a function of the platform and the Cisco IOS software release, and can be tested using the "show idp"


router#sh idb

Maximum number of Software IDBs 2000. In use 66.


Active 42 61

Inactive 2 5

Total IDBs 44 66

Size each (bytes) 4648 1400

Total bytes 204512 92400

The maximum number of interfaces (physical, subinterface, or virtual) a router can handle depends on the maximum number of SWIDBs that the router can use.

Each tunnel interface definition, such as Universal Transport Interface (UTI), Generic Routing Encapsulation (GRE), Multiprotocol Label Switching Traffic Engineering (MPLS TE), or Any Transport over MPLS (AToM) consumes an HWIDB plus one SWIDB per tunnel, plus an additional SWIDB for each additional sub-interface, for example, a Frame Relay PVC, that is tunneled. The tunnel IDBs are in addition to the original interface(s) that are tunneled.


Mohammed Mahmoud.


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